1. Field of the Invention
The present invention relates to a method and apparatus for manufacturing a sheet of safety glass, for use in motor cars or buildings, provided with a coating of plastic material for improving the safety characteristics exhibited thereby when breaking.
2. Description of the Prior Art
Sheets of safety glass known in the prior art typically comprise a glass substrate and a coating of plastic material applied on one of the two surfaces of the substrate.
When such safety glass is used as a motor car windshield, the plastic coating serves to avoid injuries to the passengers caused by sharp fragments normally produced when the glass breaks. Moreover, the plastic coating should also have particular mechanical and optical properties, such as properties of non-tearing, self-healing, resistance to abrasion and to aging, in order to meet International Standards, such as DIN, ECE, ASA standards and the like.
Compositions of and methods for producing a coating of plastic material to be applied on a glass substrate are known in the prior art.
Typically, Italian published application No. 19459A/86 describes a composition based upon polycaprolactones, acryl monomers, aliphatic polyisocyanates, flowing agents, reaction catalysts and the like selected and mixed in such a way as to form a plastic coating with the desired optical and mechanical features and able to provide the glass substrate with the required safety characteristics. A composition of this type is sprayed in a liquid state with a viscosity of 40 to 200 centipois on a glass substrate which, when such is to be used in motor cars, has been previously cut and curved to the desired configuration.
Immediately after spraying, the glass substrate is rotated horizontally at a speed of 10 to 50 rpm, so as to facilitate a levelling of the liquid film in which a uniform thickness of the film is maintained over the entire surface of the substrate.
The polymerization of the sprayed liquid takes place directly on the substrate to which the coating intimately adheres and successive infrared and ultraviolet irradiation treatments help complete the polymerization.
In a single spraying operation, it is advisable to lay a film having a thickness of 80 to 150 microns. Further, spraying and polymerization operations can be carried out to provide a coating thickness that is sufficient to provide the glass with the desired safety characteristics.
In the manufacture, on a commercial scale, of safety sheets of glass provided with a plastic coating having high optical and mechanical quality, typically resulting in a composition obtained by a process as previously described, a problem of ensuring a sufficient centrifugal force to the liquid layer during the treatment thereof by infrared and ultraviolet irradiation arises and, moreover, it is necessary to repeat the coating and polymerization process a number of times until the desired thickness is obtained without affecting the production time.